Light sensitive electric discharge device



1943- E. D. M ARTHUR 2,434,622

LIGHT SENSITIVE ELECTRIC DISCHARGE DEVICE Filed Sept. 24, 1942 2 Sheets-Sheet 1 Fig. 2.

GE TTE R SUPPORT 1 ML, I3 .3

PHOTO-IEMI-SSIVE PHOT0-EMI$8IVE SURFACE 7 SURFACE I4 I I 23 I4 2/ a; k i I 9 24 F334 q M o o O O O E Y Inventor.

j Eimer D. Mc Arthur;

Hi At'b orney Jan. 13, 1948. E. b. MCARTHUR 2,434,622

LIGHT SENSITIVE ELECTRIC DISCHARGE DEVICE Filed Sept. 24, 1942 2 Sheets-Sheet 2 "BREAKDOWN LIGHT SOURCE OFF} z -laov. vans 657 TE R JUPPOR T 64 I |'l I Inventor:

y rI i Att'orney Patented Jan. 13, 1948 LIGHT SENSITIVE ELECTRIC DISCHARGE DEVICE Elmer D. McArthur, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application September 24, 1942, Serial No. 459,532

20 Claims. 1

My invention relates to light sensitiveorphotoresponsive electric discharge devices.

Heretofore, in photo-sensitive or photo-responsive circuits Where it has been desired to efiect a circuit controlling operation'in response to light intensities, it has frequently been necessary to employ a conventional type photo-emissive electric discharge device with a highly sensitive voltage or current means, such as an electro-magnetic relay, in order that sufiicient actuating current or power is obtained to carry out the intended circuit controlling operation. For example, where it has been desired to actuate ame'c'hanism of appreciable size, it has frequently been necessary to employ a highly sensitive electromagnetic t'ype relay or equivalent device, resulting in a relatively expensive system which laclzed'the desired degree of flexibility in control 'or adjustment.

It is an object of .my invention to provide new and improved light sensitive control systems.

It is a :further object of my invention to pro- Vide new and improved photo-emissive or light sensitive electric discharge devices of the type employing ionizable mediums.

It is a still further object of my invention to provide new and improved electric discharge devices of the photo-emissive type wherein a high degree of sensitivity is obtained by electrode construction and configuration such that the thermal effect of the thermionic cathode upon the photoemissive control member is substantially negligible.

It is a still further object of my invention to provide new and improved light sensitive or photoemissive electric discharge devices of the controlled type which employ an .ionizable medium and which are capable of operation in a manner so that the anode current is accurately controllable or adjustable within a region not effecting ionization of the medium and which also permits conduction of current between an anode and 'a cathode by .an arc discharge when either thein tensity of the light impinging on the light sensitive surface attains a predetermined value, or when the control member voltage is raised with respect to the cathode.

Briefly stated, in the illustrated embodiments of my invention I provide new and improved electric discharge devices of the light sensitive or photo-emissive type comprise an anode, I

a cathode and a photo-emissive or light sensitive electrostatic control member enclosed within an atmosphere of an ionizable medium such as an inert gas. The electrode configuration is such that the thermal efiects of the thermionic oath- 2 ode upon the photo-emissive control member are substantially minimized, thereby not appreciably affecting the emission characteristics of the control member. In this manner a greater sensitivity of control is obtained than that afforded by arrangements -provided heretofore.

In accordance with a stil1 further feature of my invention, I provide in light sensitive electric discharge devices of the above described type a heat shield which may be metallic and which is positioned between the thermionic cathode and the photo-sensitive control member, thereby absorbing a large percentage of the radiation and convection heat incident to the thermionic cathode and rendering the control member emission characteristics substantially independent of cathode temperature.

In accordance with a still further feature of my invention, I provide new and improved control circuits for electric discharge devices of the above-described type whereby appreciable power or current is available for the actuation of electromagnetic devices such as relays, etc. In accordance with a still further feature of my invention, I provide new and improved light sensitive electric discharge devices of the controlled type having a photo-emissive electrostatic control member the potential of which permits control or adjustment of the'anode current for given values of light intensity impinging on the control member within a region of operation which does not effect appreciable ionization of the medium surrounding the electrodes. The potential of the electrostatic control member may also be controlled so that the medium does become ionized to establish an arc discharge between the anode and cathode in the event the intensity of the light impinging on the photo-emissive surface of the control member attains a predetermined value.

For a better understanding of my invention, reference may be had to the following descriptlon taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims. Figs. 1 and 2 diagrammatically illustrate one embodiment of my invention which is a light sensitive electric discharge device of the controlled type; Figs. 3 and 4 diagrammatically illustrate alternative forms or configurations of the photo-sensitive electrostatic control member; Fig. 5 represents diagrammatically a light-sensitive system embodying one feature of my invention wherein the anode current of the discharge device may be controlled or adjusted within two separate regions, that is, within a region where ionization of the medium isobtained and a second region wherein ionization is not produced. Fig. 6 represents certain operating characteristics of my invention, particularly the system shown in Fig. 5. Figs. 7-10, inclusive, diagrammatically illustrate still further light-sensitive systems embodying features of my invention; Fig. 11 diagrammatically represents a still further embodiment of my invention wherein the electrode configuration is of diiierent form than that illustrated in Figs. 1 and 3, and Fig. 12 illustrates a still further modification wherein a mesh type anode and a heat shield are provided.

Referring now to Figs. 1 and 2 of the drawings, my invention is there illustrated as comprising a light-sensitive or photo-sensitive electric discharge device of the controlled type including a plurality of electrodes mounted within a glass receptacle or bulb I mounted on a base member 2 and which acts as a support for the discharge device as a whole. The bulb I may contain an ionizable medium, such as an inert gas at the desired low pressure, and this operation may be effected by filling the exhausted bulb with a gas through a suitable glass tubulation (not shown) which is sealed off and which may lie Within the base 2. For example, I may employ an atmosphere of argon at a pressure of substantially 100 to 150 microns. It will be appreciated that I may employ other inert gases such as neon and zenon. etc. if desired. I provide within the enclosure of the bulb I a thermionic cathode 3 of elongated form and which may be constructed of an electron emissive material such as tungsten. thoriated tungsten, or the like. Alternatively, the cathode 3 may be constructed of a metal, such as nickel, which is coated with an alkaline earth metal 4 or an oxide thereof to increase the electron emissivity. A cathode heating-element (not shown) extends into the interior of the cylindrical cathode 3, and cathode heating-element terminals 5 and 6, also shown in Fig. 2, are provided. A glass press 1 supports and seals conductors 8 and 9 which are connected to terminals 5 and 6. A cathode conductor l is'also supported by and sealed in the glass press 1 and is terminated in the metallic prong or conductor H which is also attached to the base 2.

I provide an anode I 2, which may be of elongated configuration, such as a metallic cylinder or rod having its principal dimension or length substantially parallel with that of the thermionic cathode 3 and which is of relatively small area. A conductor and supporting member l3 for the anode I2 is supported by and sealed in the glass press 1 and is terminated in an externally accessible conductor l3 attached to the base 2. Of course, it is to be appreciated that other forms of anode structure may be employed.

In order to control the anode current and to provide means which is light-sensitive, I provide a photo-emissive electrostatic control member 14 which is connected in spaced relation between the cathode 3 and the anode I2 and may be supported by a stem or conductor which is in turn supported by and sealed in the glass press T and made externally accessible by means of a conductor I6 which is supported by the base 2. A rigid construction of the control member 14 and the supporting means therefor may comprise supporting members I! and I8 which may be welded or soldered to the control member l4 and one of which, such as supporting member 18, may be attached or welded to conductor l5 and the other of which may be supported by the member i9 which is embedded or anchored in the glass press 1.

The photo-emissive electrostatic control member [4 may be coated with a material such as barium or caesium to increase the emissivity thereof. For example, the electrostatic control member l4 may be photosensitized by using a photo-sensitive surface, such as caesium, placed on an oxide film upon a silver base. Of course, other photo-emissive metals or alloys, such as a1- kaline earth metals and oxides thereof, may be employed. The control member [4 in one modification of my invention is preferably of semicylindrical configuration being concave to the: anode l 2 and convex toward the cathode 3. Furthermore, the area of the control member 14 is great as compared with that of the cathode 3 so that the thermal efiects of the thermionic oath-- ode 3 upon the emission characteristics and control characteristics of the control member l3 aresubstantially negligible. Stated in other words, the projected area of the cathode 3 upon the su1'-- face of the control member is very small.

The light sensitive surface of control member [4 may be placed upon the concave portion of control member l4 facing the anode l2 and may be deposited thereon in the usual or conventional manner. One detail view of the form of the control member 14 is illustrated in Fig. 3 where the portion of the control member nearest the cathode 3 is provided with an elongated aperture 20 which is substantially co-extensive with the principal dimension of the cathode 3. When such a configuration of the control member is employed, it will be appreciated that some means is required to support the two sections of the control member and to accomplish this result I may employ semicircular conductors 2| and 22 which are Welded or soldered to the control member 14.

An alternative form of the photo-emissive electrostatic control member is shown in Fig. 4 wherein a semicylindrical control member 23 is provided with a series of vertically spaced apertures 24 in a line which is substantially co-extensive with the principal dimension or height of the thermionic cathode 3.

It will be appreciated that in both of the arrangements illustrated in Figs. 3 and 4, the projected area of the thermionic cathode on the photo-sensitive control member is relatively small, approaching zero as a limit in the arrangement shown in Fig. 3. In this manner, the thermal effects of the cathode on the control member are minimized, and at the same time there is provided an appreciable path for the arc discharge which is established under certain conditions of operation between the anode i 2 and the cathode 3.

The operation of the embodiment of my invention illustrated in Figs. 1 and 2 may be more fully appreciated by referring to the circuit arrangement shown in Fi 5 wherein the photo-emissive or light sensitive electric discharge device 25 is diagrammatically illustrated and may be constructed to comprise the structural features described above with particular reference to Figs. 1 and 2. In the arrangement of Fig. 5, the electric discharge device 25 is illustrated as comprising an anode 26, a thermionic cathode 21 having a heating element 28, and a photo-emissive electrostatic eontrol member 29 which may be subjected to light of different intensities derived from a source which is diagrammatically illustrated as comprising a lamp 30.

The cathode heating element 28 may be energized from any suitable source of current such as vice 36 do ich 'imp'i s on control V i wjr i nv si g "pqtntim of nhree s illustrated in Fig fi,

'b'iigrhizfteachs a value intensity remains above this predetermined value, current will be transmitted to the actuating coil 43 during each positive half cycle of voltage ap: plied to anode 38, and the capacitance 46 serves to maintain the current through the actuating coil substantially constant even during negative half cycles. However, if the light intensity decreases to a value below that which effects the re-initiation of arc discharges during each positive half cycle, the anode-cathode current flow will not continue and the charge of capacitance 46 will be dissipated causing coil 42 to be deenergized or suificiently reduced in energization, thereby opening contacts 44.

A still further modification of my invention is illustrated in Fig. 8 which is similar in many respects to that shown in Fig. 7 and corresponding elements have been assigned like reference numerals. In the modification of Fig, 8 a separate source of biasing potential, such as an alternating current source 49, is employed to impress a biasing potential across control member 39 and cathode 3'! through a transformer 50. The alternating bias voltage is preferably adjusted to be 180 electrical degrees out of phase with the anode-cathode voltage, and the secondary winding of transformer 50 is connected in series relation with a capacitance During each inverse half cycle of anode-cathode voltage, the voltage produced by transformer 5!! establishes a biasing potential on control member 39 by charging the capacitance 5|, the current for producing this charge being obtained by virtue of the electron emission current which flows between control member 39 and cathode 31. If the intensity of the light increases sufiiciently to increase the photo-emissive current to a value which efiects ionization of the medium and establishes an arc discharge when the anode 38 becomes positive during its cyclic variation in potential, anode-cathode current will fiow effecting energization of the load device which may be actuating coil 4'2. In this manner, it will be appreciated that the negative biasing potential is established during each negative half cycle of anode-cathode voltage. The discharge of capacitance 5! facilitates establishment of the are discharge within device 36. This type of operation will be eifected when the increase of light intensity occurs during a positive half cycle or during a negative half cycle of applied anodecathode voltage. When the initiation of the increased light intensity occurs during a negative half cycle, the ionization effect is sufficient to carry over into the succeeding positive half cycle so that current flow is produced in the anodecathode circuit.

A further embodiment of my invention is illustrated in Fig. 9 which is similar in many respects to Fig. 7, and correspondingelements have been assigned like reference numerals. Instead of employing an alternating current circuit for establishing the voltage difference between the cathode 3'! and the control member 39, I may employ a unidirectional source such as a battery 52. In this arrangement, when the light intensity attains a predetermined value, an arc discharge will be initiated and current flow will be established in the anode-cathode circuit. The modifications of my invention shown in Figs, 5 and ,7-10, inclusive, are described and claimed in divisional application S. N. 553,528 filed September 11, 1944, and assigned to the assignee of the present application, I One of the principal features of light sensitive electric discharge devices built in accordance-with my invention is that the control circuit which establishes the sensitivity of the initiation of operation may be a low impedance circuit and not one which functions principally in response to potential. By the provision of such a device and system, it is possible to obtain greater degrees of sensitivity than that afiorded by many of the prior art arrangements, particularly where it is desired to employ the device and the system in a climate which is subjected to extreme variations in atmospheric conditions such as variations in relative humidity and temperature which would tend to produce condensation of moisture on high voltage elements comprising the sensitive part of the circuit, thereby reducing its sensitivity.

In Fig. 10 I have diagrammatically illustrated a still further embodiment of my invention wherein a low impedance control circuit 53 is connected across the control member 39 and cathode 3'! of electric discharge device 36, the initiation of the arc discharge or operation of the device being effected by the current which flows between control member 39 and cathode 31. That is, the circuit is primarily a current responsive circuit and does not depend appreciably upon large voltage variations, thereby dispensing with the em ployment of impedance elements of relatively high value which in turn effect or cause the presence of relatively high voltages.

Fig. 11 diagrammatically illustrates a still further modification of my invention, particularly a new and improved light sensitive electric discharge device which employs an ionizable medium such as an inert gas, and a plurality of electrodes including an electrostatic photo-emissive control member. Referring particularly to the electric discharge device shown in Fig. 11, it comprises an enclosing receptacle such as a glass bulb 54 which may include a cylindrical type glass press 55 which is sealed at the desired pressure. The glass bulb 54 may be evacuated and gas filled through a tubulation (not shown). The bulb 54 is supported by a base 56. I provide a. photo-emissive electrostatic control member 58 which surrounds an appreciable dimension of a wire anode 51 and which may be an inverted truncated-cone having a photo-emissive surface on the concave portion thereof facing the anode. The control member 58 is constructed to have a lower opening 59 through which the anode 51 extends Anode 51 and control member 58 may be supported above the glass press 55 by uprights 60, GI and 62, respectively. Anode 51 and upright 69 may be supported by a vitreous or glass sleeve or pencil 60' in which these members and conductors 65 are embedded or sealed, The up.- rights 60 and 6! are connected to prongs 63 and 64 through conductors 65 and 56, respectively, which may be sealed in the glass press 55, V I provide a thermionic cathode preferably constructed of a metallicribbon 61 having thermionic emissive characteristics and which may have asubstantially circular configuration lying in a. plane substantiall perpendicular to the principal axis of the anode 51. The cathode 61 may be supported by wire uprights 68 and 69 which, in turn, are sealed in the lass press 55 and made externally accessible by prongs or conductors 10 and H which are supported by base 56. Itwill be noted that the projected area of cathode ribbon 51 upon the surface of the control member 58 is inappreciable so that the thermal efiects of the cathode do not materially afiect the emission characteristics of the control member 58..

arr a e? In operation; the embodiment of 5 my invention shown in Fig; 11 functions substantially the same as that explainedaboveinconnection with Figs, 1; 2 and'5.

In the modificationof my-invention shown in Fig. 12, whichis similar in many-respects to that shown in Fig; 1 1 and corresponding elements have been assignedliliereference numerals, I provide aheat shield in spaced relation between the thermionic,v

ribbon cathode 61and thephoto-emissive electro static control member 581 This heat shield may be constructed of metalsuch as nickel or coppehaving anappreciable thermal capacity to ab sorb heat which isra'diated' from the cathode 67. The heat shield may comprise a metallic cylinder 12 supported upon a pencil or glass stem. 15

and may be sealedthereto by employing analloy of iron-nickel-cobalt. The portion of the shield which extends toward the control member 58 may comprise a flared or flanged section including ametallic annular member which lies in a: plane substantially perpendicular tothe direction of heatiiow or'heat radiation between members El and 58.- Conductors T1 and 18, which are connected to anode T5 and control member 58, may extend through and be sealed in the glass stem 73; and may be made externally accessible by-mea-ns of a base (not shown).

In the modification of my invention shown in Fig. 12, I provide an anode which may take the form of an inverted cone T5 and may be constructed of a wire mesh or grid, and which is' supported at the vertex thereof by means of an upright 16 which extends through the lower opening 59 in the control member 58, The concave portion or surface of the control member 58 is also coated with a photo-emissive material and the initiation of current between the anode and cathode may be accomplished in the same manher, as explained above, in connection with the devices shown in Figs. 1, 2, 5 and '11.

While I have shown and described my invention as applied to particular devices embodying various elements diagrammatically shown, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention; and I, therefore, aim in the app-ended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A light sensitive electric discharge device comprising a plurality of electrodes within an enclosing receptacle, said electrodes comprising an elongated thermionic cathode, an elongated anode and a photo-emissive electrostatic control member of substantially semi-cylindrical configuration in spaced relation between said cathode and said anode with the open side of said member towards said anode and provided with at least one aperture therein to permit the establishment of an electric discharge between said cathode and said anode.

2. A light sensitive electric discharge device including a plurality of electrodes enclosed in an atmosphere of an inert gas, said electrodes comprising an elongated cylindrical thermionic cathode, an elongated cylindrical anode and a photoemissive electrostatic control member in spaced relation between said anode and said cathode and having a convex curvature toward said cathode, said control member having a substantial por-' tion of the area in alinement with said anode and said cathode cut away.

32 A1 light sensitive electric discharge device.

comprising a plurality of electrodes, within, an

atmosphere: of; anuinert gas, said. electrodes com-,

prising an, elongated cylindrical anode, a cathode in spaced-' relation: with said anode and a photo-i-emissivei electrostatic control member of substantiallytruncatedzoonicalzconfiguration substantiallysurrounding said anode,- the projected area ofsaid cathode, upon said" control member being substantially negligible:thereby minimizing, tnezthermal'eileot ofiisaidthermionic cathode upon: the; emission characteristics. of said control membersv 413A; light: sensitive, electric discharge device comprising a; plurality of enclosed electrodes;

within-- an; atmosphere of an inert gas, said electrodes comprisingan elongated: anode, a thermi- 0111C cathode: in; spaced relation with said anode, aphotwemissive; electrostatic control member of substantially truncated; conical configuration sur-i rounding an. appreciable length of said anode, the projected area. ofisaid cathode upon said control:' member being-substantially negligible reb'y min-imizing thethermal; effects of said cathode upon the emission characteristics of said control member.

51A light. sensitiyte electric discharge device comprising, a: plurality of enclosed electrodes within an atmosphere of an inert gas, said electrodes. comprising an. elongated anode of relatively small cross sectionalarea, a thermionic cathode; comprising a: substantially circular cathode in spaced relationwith said anode, the plane of said cathode being substantially perpendicular to. the'principalaxis of said anode, a photoi-emissivev electrostatic control member of substantially truncated conical configuration surrounding a substantial portion of the length of saidano'de and being of relatively large area so that the projected area of said cathode upon said control member is substantially negligible thereby minimizing the thermal effects of said cathode thereon.

6. A light sensitive electric discharge device comprising a plurality of enclosed electrodes within an atmosphere of an ionizable medium, said electrodes including an anode of relatively small cross sectional area, a thermionic ribbon cathode'ofsubstantially circular configuration lying in a plane substantially perpendicular to the principal axis of said anode, a photo-emissive electrostatic control member in spaced relation between said'anode andsaid cathode and being of substantially truncated conical configuration surrounding an appreciable portion of the principal axis of said anode, and a metallic heat shield in spaced relationbetween said cathode and control member to minimize the thermal effects of said cathode on the emission and control char,- acteristics of said control member.

7. A light sensitive electric discharge device comprising a plurality of enclosed electrodes in eluding a thermionic cathode, an anode and a photo-emissive electro'staticcontrol member, said control member comprising a, member having a photo-emissive surface facing said anode and providedwit'h an aperture toafiord a path for the establishment of an electric discharge between said cathode and said anode, and a heat shield in spaced relation between said cathode and said control member.

:8. A light sensitive electric discharge device comprising a jpluralityioi enclosed electrodes in c'luding :a. thermionic cathode and a mesh-type anode, providing an electron discharge path 11 therebetween and a photo-emissive electrostatic control member in spaced relation from said anode and substantially surrounding it.

9. A light sensitive electric discharge device comprising a plurality of enclosed electrodes including a thermionic cathode and a conical mesh anode, providing an electron discharge path therebetween, and a photo-emissive electrostatic control member of truncated conical shape substantially surrounding said anode.

10. A light sensitive electric discharge device comprising a plurality of enclosed electrodes including a thermionic cathode, a conical mesh anode and a photo-emissive electrostatic control member of truncated cone shape substantially surrounding said anode and having a photoemissive surface facing said anode.

11. A light sensitive electric discharge device comprising a plurality of enclosed electrodes including a thermionic cathode, an anode of conical configuration, a photo-emissive electrostatic control member of truncated conical configuration substantially surrounding said anode and having a photo-emissive surface facing said anode, and a heat shield in spaced relation between said cathode and said control member.

12. A light sensitive electric discharge device comprising a plurality of enclosed electrodes including a thermionic cathode, an anode, and a photo-emissive electrostatic control member in spaced relation between said anode and said cathode and having an aperture through which a discharge between said anode and said cathode may pass, said cathode having opposed to said control member an area the projection of which on said control member is very small compared to the total area of said member whereby the exchange of thermal energy between said cathode and said control member is minimized, said control member having a generally enlarged crosssection at the end thereof opposed to said anode, a generally small cross-section at the end thereof opposed to said cathode and a wall portion therebetween having a surface extending at an angle with respect to the path of radiation between said cathode and said member whereby the exchange of thermal energy between said cathode and said member may be further minimized.

13. A light sensitive electric discharge device comprising a plurality of enclosed electrodes including a thermionic cathode, an anode, and a photo-emissive electrostatic control member in spaced relation between said anode and said cathode and having an aperture through which a discharge between said anode and said cathode may pass, said cathode having opposed to said control member an area the projection of which on said control member is very small compared to the total area of said member whereby the exchange of thermal energy between said cathode and said control member is minimized, said control member having a generally flared configuration having an enlarged open end thereof facing said anode, a smaller end thereof containing said aperture facing said cathode and a wall portion between said ends having a surface extend.. ing at an angle with respect to the path of radiation between said cathode and said member whereby the exchange of thermal energy between said cathode and said member may be further minimized.

14. A light sensitive electric discharge device comprising a plurality of enclosed electrodes includin a thermionic cathode, an anode, and a photo-emissive electrostatic control member in 12 spaced relation between said anode and said cathode and having an aperture through which a discharge between said anode and said cathode may pass, said cathode having opposed to said control member an area the projection of which on said control member is very small compared exchange of thermal energy between said cathode and said control member is minimized, said control member having a generally curved surface with a concave side opposed to said anode and a convex side opposed to said cathode whereby the exchange of thermal energy between said cathode and said member may be further minimized.

15. A light sensitive electric discharge device comprising a plurality of enclosed electrodes including an elongated cylindrical thermionic cathode, an elongated cylindrical anode, and a substantially semi-cylindrical photo-emissive electrostatic control member in generally parallel spaced relation between said anode and said cathode and having an aperture through which a discharge between said anode and said cathode may pass, said cathode having opposed to said control member an area the projection of which on 'said control member is very small compared to the total area of said member whereby the exchange of thermal energy between said cathode and said control member is minimized, said control member having the concave side thereof opposed to said anode and the convex side thereof opposed to said cathode whereby the exchange of thermal energy between said cathode and said member may be further minimized.

16. A light sensitive electric discharge device including a plurality of electrodes enclosed within an envelope, said electrodes comprising an elonated cylindrical thermionic cathode, an elongated rod-shaped anode, and a photo-emissive electrostatic control member of substantially semi-cylindrical form positioned between said anode and said cathode with the convex side thereof nearer to said cathode and provided with at least one opening having an unobstructed area between said anode and said cathode which is a substantial portion of the projected area of said cathode on said control member.

17. A light sensitive electric discharge device including a plurality of enclosed electrodes, said electrodes comprising an anode and a thermionic cathode, providing an electron discharge path therebetween, and a photo-emissive control member between said anode and said cathode in spaced relation thereto, and a heat shield between said cathode and said control member to minimize the thermal effects of said cathode upon said control member.

18. A light sensitive electric discharge device comprising a plurality of enclosed electrodes, within an ionizable medium, said electrodes comprising an anode and a thermionic cathode, providing an electron discharge path therebetween, and a photo-emissive electrostatic control member between said anode and said cathode in spaced relation thereto, and a metallic heat shield between said cathode and said control member to minimize the thermal effects of said cathode upon the emission characteristics of said control member.

19. A light sensitive electric discharge device comprising a plurality of enclosed electrodes, within an ionizable medium, said electrodes comprising an anode and a thermionic cathode, providing an electron discharge path therebetween,

and a photo-emissive electrostatic control mem-- her between said anode and said cathode in spaced relation thereto, and a flanged metallic heat shield in spaced relation between said control member and said cathode to minimize the thermal effects of said cathode upon the emission characteristics and the control characteristics of said control member.

20. A light sensitive electric discharge device comprising a plurality of electrodes including an anode, a cathode in spaced relation with said anode and a photoemissive electrostatic control member of substantially truncated conical configuration substantially surrounding said anode, the projected area of said cathode upon said control member being substantially negligible thereby minimizin the thermal effect of said thermionic cathode upon the emission characteristics of said control member;

ELMER D. McARTI-IUR.

REFERENCES CITED The following references are of record in the file of this patent:

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